The present invention relates to a process for drying young grass, green fodder, seaweed and similar products having a high moisture content by forcing as far as possible uniformly distributed air through a stationary, relatively thick bed of the goods to be dried, while using first non-heated air and towards the end of the drying period air that has been heated to reduce its relative humidity to 35% or less. The invention also relates to an apparatus for carrying out such a process, said apparatus comprising a grid surface or the like on which the goods to be dried are placed, and a blower forcing possibly heated air up through the grid surface and through the grass.
It has long been known that by artificially drying young grass a valuable fodder which may substitute all kinds of concentrates for domistic animals, especially cattle and sheep, can be produced. Firstly, young grass has a high content of various types of proteins in digestible form and having a favorable distribution between the types thereof. At the same time, the content of vitamins and provitamins, especially carotene, is of importance. When the drying process is such that the natural content of protein and carotene in the grass is maintained, a feeding on dried grass will keep the animals in good shape.
Fodder from young grass may be of particular importance in Norway where the climate is favourable for the production of grass, with respect to both precipitation, temperature and light conditions. The production of dried grass can therefore give a larger yield per unit area than other crops of fodder products. This is also due to the fact that when the precipitation is suitable, a more abundant manuring is possible if the grass is cut several times during the season.
Drying of young grass entails advantages compared with ensilaging of the grass. Thus, the concentration of the fodder is higher and fermentation is avoided. In addition, the use of acid and the pollution due to the silage juice is eliminated. Further, ensilage should be avoided when the milk is to be used for making high quality cheese. However, when using dried grass, the resulting milk is very well suited for producing butter and high quality cheese.
In spite of the above strong motives for the use of dried grass, the drying of young grass has hitherto not become common practice. This is due to the difficulties attached to the provision of a rational and economic drying of the grass at a rate which is suitable in farming. Young grass consists of relatively large amounts of leaves and small amounts of stems as compared with fully grown grass (hay). The dried grass bed will therefore become relatively dense, and fermentation and putrifiction very often occur upon storing. To keep its quality the dried young grass must, therefore, be dried down to a moisture content of approximately 12%, whereas in hay a moisture content of up to 20% is acceptable, especially if airily stored. Regard being had to the fact that the newly cut, young grass has a very high moisture content of approximately 80% of its weight, relatively much water has to be removed by the drying. In addition, the grass is so short that it is not easily framed in the same way as hay. Drying is therefore usually carried out by means of heat in apparatus which are rather expensive, complicated and/or labour consuming, and in which i.a. the transportation of the raw grass plays an important role. A plurality of various drying apparatus are known, such as drum driers, belt driers, shuffle driers etc. usually involving a movement of the goods to be dried. The heat consumption is rather high, often from 1000 calories or more per kg evaporated water, corresponding to approximately 4 kWh per kg finished dried grass. Such an energy consumption prevents an economic production of dried grass.
As the drying air is passed through the goods to be dried it gradually cools and absorbs moisture. After having passed through a drying zone which is usually 20 to 30 cm, the drying air will have such a high humidity that it is no longer capable of absorbing moisture from the grass. The end of this drying zone may be called the drying front. As long as the moisture content in the grass is above approximately 65%, the drying air becomes saturated with moisture and the drying front lies within the grass bed. At first, only the inner zone of 20 to 30 cm of the grass will be dried. As the moisture content in this zone is reduced, the drying front will gradually advance through the goods to be dried.
A drying process comprising a preliminary drying by means of non-heated or only slightly heated air followed by a final drying with a stream of somewhat heated air gives a better economy than drying with heated air from the beginning. This is due to the fact that the reduction of the relative humidity of the drying air by heating is not necessary as long as the humidity of the outdoor air is not substantially above 70% and the water content of the grass is relatively high. However, experience has shown that when the moisture content in the grass at the end of the drying process becomes relatively low, drying air with a relative humidity of 30 to 35% must be used to permit the water content in the grass to be reduced to approximately 12% during a reasonable span of time. This reduction of the relative humidity of the air is suitably obtained by a certain heating of the air. In order to avoid the requirement of too large heating units, the rate of air through the grass can be reduced.
In the publicly available Norwegian patent specification No. 135 644 it has been suggested an apparatus for drying young grass in several steps while using a plurality of stationary sections. This apparatus consisted of a ventilated storage place in the form of one or several covered bins for preliminary drying with unheated air. Thereafter, the grass was to be transferred to a drying apparatus with circulating heated air for completing the drying, so that the storage place could once more be filled with a new lot of newly mown grass. This is then subjected to a preliminary drying at the same time as the drying of the first lot of grass is completed in the drying apparatus. Such a process requires relatively much labour due to the transfer of all the grass. The apparatus is relatively large and expensive.
It has also been suggested (see NO-PS 108 897) to dry young grass in "drying sheds" having a grid-formed sloping floor through which drying air is passed by means of a blower at the rear side of the "shed". Such a drying shed is relatively simple and labour-saving. It is, however, still a stationary apparatus.
In the abovementioned apparatus it has further proved difficult to avoid that the drying air will find its way along the walls of the drying room rather than being distributed through the goods to be dried. In addition, the heating economy is still not satisfactory.